Killing Floor is a first person shooter, allowing each player to move through a 3D environment. Gameplay consists of two game types: Killing Floor and Objective Mode. In Killing Floor Mode, the player fights waves of zombie-like specimens, or ZEDs, with each wave becoming successively more difficult, until it concludes with a battle against a "boss" specimen, called the Patriarch.[5] Objective Mode poses different challenges in addition to fighting against the zombie specimens. Players earn in-game money for each kill, as well as for surviving to the end of a wave. In the time between each waves, players can visit a trader to buy and sell ammo, armor, and additional weapons, such as katanas, pipe bombs, flamethrowers, shotguns, and so forth; the Trader's location on the level varies at the end of each wave, discouraging players from camping in one location. Weapons may also be found randomly across the level. Players are encouraged to work together; they can trade items and drop money, healing is more effective on other players than on oneself, and the team can strategically weld doors shut to provide a temporary barrier from the oncoming horde while funneling the other creatures to specific areas.

Players select one of seven perks at the start of a match and between rounds. The perk, similar to the idea of a character class, provides bonuses towards certain weapon types, armor, movement, and other factors. Players can level up each perk to increase the benefits it offers. For example, the "Sharpshooter" perk, which provides accuracy benefits when using scoped rifles, can be raised in level by completing a number of headshots with specific weapons. The player does not need to play with that perk to raise its level. For example, the Field Medic perk requires the player to heal a number of points to level up, but this can also be done while playing any of the other perks.

As long as at least one member survives the wave, players that die during a wave will drop their current weapon and re-enter the game at the start of the next wave. If the entire squad is wiped out, they will have to restart the game from the first round.

Each game can be configured to alter the length and difficulty of the various waves. The number of creatures in the enemy waves are based on the number of players in the game, increasing in numbers if more players join the game in progress. All but the weakest enemies have some special attack that can inflict significant damage on a player. A feature called "zed time" awards particularly gory kills, such as headshots, with slow-motion, even in multiplayer mode.

Alex Quick, the lead level designer and texture artist, stated that "there are a whole bunch of ideas on the drawing board that we can add in after launch, including the 'Story' mode from the mod, for instance."[6] A software development kit (SDK) and level editor are included to aid in the creation of modifications and levels.

A genetically manipulated and cloned human being as they appear in the game, showing grotesque mutations.

The game takes place in London, England. Horzine Biotech, a biotechnology company, is contracted to conduct experiments of a military nature involving mass cloning and genetic manipulation. Something goes horribly wrong during the process of the experimentation, and human subjects begin to exhibit grotesque mutations and disfigurement. They become increasingly hostile, and eventually overrun the internal security forces of the corporation.

Hours later, the first waves of the specimens break out onto the surface, disrupting a peace protest outside the well-known military contractor. Despite the best efforts of the local police, the civilians are quickly overwhelmed and consumed by the seemingly endless supply of clones now streaming from the gaping maw of Horzine Biotech's headquarters. Having escaped their sterile prison, the creatures begin to fan out to neighboring areas, devouring the helpless citizens of London while the Metropolitan Police fruitlessly attempt to fight back.

Desperate to contain the outbreak from reaching overseas as planned by the mutated and cybernetically-enhanced scientist Dr. Kevin "The Patriarch" Clamely, the British government quickly begins to organize ragtag teams of surviving British Army soldiers and Special Branch police officers to fight back against the hordes of mutated "specimens." The player takes the role of a member of one of these teams as they partake in a variety of missions in and around the city of London.

Killing Floor was originally a total conversion mod for the game Unreal Tournament 2004, first released in 2005.[7][8] The lead developer Alex Quick was first approached by Tripwire Interactive about porting Killing Floor to their game Red Orchestra: Ostfront 41-45, but Quick did not follow through. A few years later, when their user base started to dwindle, Quick contacted Tripwire again to negotiate the port to Red Orchestra, and its distribution via Steam—similar to what Tripwire had already done with another Red Orchestra mod Mare Nostrum—and then later, the retail game.[6] The mod version had its fifth and final official update in July 2008.

The full retail version was announced in March 2009. With the assistance of the original mod team, Tripwire would be the new developer.

In 2012, with the launch of the Linux Steam version, the original non-Steam servers were closed and the game was made a Steam exclusive.[9]

Killing Floor was released on 14 May 2009 after about three months of work by a crew of around ten developers.[10]

A late 2012 patch introduced support for the Steam Workshop; a distribution platform for user-made content functioning via the Steam client.[11] In mid-2013, a new mode called "Objective Mode" was added, where the player team would be tasked with completing specific objectives over the course the various waves in addition to outright survival.[12]

Killing Floor was the top-selling game on Steam shortly after it was released (as of May 19, 2009).[18] During the PlayStation Experience event in December 2014, Tripwire Interactive announced that Killing Floor had sold almost 3 million copies on PC.[19]

Killing Floor has won a number of awards including Overall Best PC Game of 2009 in Voodoo Extreme's Reader Choice Awards. It also won 2 other categories in Voodoo Extreme's Reader's Choice Awards: Best PC Shooter of 2009 and Best PC Game Expansion or DLC of 2009.[20]

Eurogamer commented that "though not perfect", it was "clever and relatively cheap".[15]GameSpot commented that the game graphics compared poorly with contemporary shooters, and that the voice acting was bad, but considered that the sound effects were very good, and that despite its flaws, "you can't help but enjoy it."[16] Some reviewers also considered that the lack of any real plot or aim for the players other than killing specimens, the small number of existing maps, and the repetitiveness of the gameplay reduced its replay value.[21][22][23]Killing Floor has a Metacritic score of 72/100.[14]

^ ab"Killing Floor Review". GameSpot. May 26, 2009. Visually, Killing Floor compares poorly with contemporary shooters, thanks to its five-year-old engine(...)The sound effects are outstanding, especially for guns, and you can often use subtle sound clues to detect incoming enemies. Unfortunately, the voice acting and soundtrack don't achieve the same level of quality(...)Meanwhile, derivative, generic metal music does little to improve the tone(...)It's absurd and crass, and yet you can't help but enjoy it